Wavelength division multiplexing optical transmission system

ABSTRACT

In a wavelength division multiplexed optical transmission system wherein the zero dispersion wavelength of the optical fiber transmission path  224  is in the 1550 nm region, among multiplexed optical signals, the wavelengths of either of at least two optical signals are allocated between 1450 nm and 1530 nm, or between 1570 mn and 1650 nm.

FIELD OF THE INVENTION

The present invention relates to a wavelength division multiplexedoptical transmission system which transmits a wavelength divisionmultiplexed optical signal using a dispersion-shifted fiber.

DESCRIPTION OF RELATED ART

Wavelength Division Multiplexing (WDM) transmission technology is atechnology in which optical signals of differing wavelength (opticalfrequency) are multiplexed, and transmitted via one optical fibertransmission path. Here, the optical signal is the optical output of alight source directly modulated by a data signal (direct modulationtype), or an optical transmission wave output from a light sourcemodulated by a data signal using an external modulator (externalmodulation type), and this wavelength is determined by the light sourcewavelength.

By disposing along the optical fiber propagation path optical amplifierswhich amplify the optical signal as-is, and compensating thetransmission loss of the optical fiber transmission path, it is possibleto extend the span between regenerative repeaters which are necessaryfor discriminative reproduction processing at the electrical step. Thisoptical amplifier can increase the transmission capacity of an installedoptical fiber transmission path by many times the number of wavelengthssimply by altering the transmission and receiving apparatuses forwavelength division multiplexing use because it possesses a function inwhich optical signals of differing wavelength are amplified together.For example, the amplification wavelength bandwidth of an erbium dopedoptical fiber amplifier (EDFA) is between 1.53 μm and 1.56 μm, and bymultiplexing optical signals at wavelength intervals of 0.8 nm in thiswavelength band, about 30 channels of optical signals can be transmittedthrough in one optical fiber.

However, installed dispersion-shifted fibers transmit optical signals ofa designed zero-dispersion wavelength. When transmitting wavelengthdivision multiplexed optical signals in this dispersion-shifted fiber,cross-talk due to four-wave mixing, a non-linear optical effect, isgenerated, and because of this the input power to the transmission pathfiber could not be increased. In the following this problem will beexplained in detail.

The propagation loss of a silica optical fiber is minimal in the 1.5 μmto 1.6 μm region. A dispersion-shifted fiber is designed so that thewavelength dispersion is zero in the 1.55 μm wavelength region, and bysuppressing waveform degradation due to wavelength dispersion at thiswavelength, the transmission distance can be increased. In addition,while the International Standards Organization has stipulated that thezero dispersion wavelength of a dispersion-shifted fiber is allocatedbetween 1.525 μm and 1.575 μm, practically the distribution is roughlybetween 1.535 μm and 1.565 μm, centered on 1.550 μm, and up to thepresent, these have been widely installed.

In contrast, when optical signals of differing optical frequencies areinput into an optical fiber, new optical frequencies dependent on thedifference in input optical frequencies are generated based onthird-order non-linearity within the optical fiber. This is called“four-wave mixing,” and is a phenomenon wherein, for example, an opticalfrequency f₁+f₂−f₃ is generated from input optical frequencies f₁, f₂,and f₃. This four-wave mixing is more easily generated the smaller thedispersion value of the input optical wavelength, or the larger theinput power of each individual wavelength.

If the optical frequency intervals between the wavelength divisionmultiplexed optical signals input into this kind of optical fiber areuniform, the optical frequency newly produced by four-wave mixing willconform with one optical wavelength among those of the optical signal,and strong noise will be generated by mutual interference. In addition,even when the optical frequency intervals of the wavelength divisionmultiplexed optical signal are not uniform, the optical power of theoriginal optical signal is consumed in the generation of four-wavemixing, and this produces strong noise; When the optical frequencyinterval of the wavelength division multiplexed optical signal has evenspacing, excess noise originating in four-wave mixing is generated by aninput power per wavelength from about −5 dBm, and when the spacing isuneven, it is generated by an input power per wavelength from about −2dBm. Because of this, the optical power that can be input into theoptical fiber transmission path cannot exceed this value, and as aresult, the transmission distance is limited.

The object of the present invention is to provide a wavelength divisionmultiplexed optical transmission system which can use dispersion-shiftedfibers installed in an optical transmission path and transmittingwavelength division multiplexed optical signals, and can increase thepermissible optical input power to a dispersion-shifted fiber.

The present invention is a wavelength division multiplexed opticaltransmission system or a wavelength division multiplexed opticaltransmission method wherein among wavelength division multiplexedoptical signals the wavelengths of either of at least two opticalsignals are between 1450 nm and 1530 nm, or between 1570 nm and 1650 nmwhen a dispersion-shifted fiber whose zero dispersion wavelength is inthe 1550 nm region is used as a transmission path.

In this manner, by limiting the used wavelength bandwidth, the influenceof four-wave mixing in the dispersion-shifted fiber can be avoided.Thus, it is possible to enlarge the permissible input power to thedispersion-shifted fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of the first embodimentin the first wavelength band limit.

FIG. 2 is a block diagram showing the construction of the secondembodiment in the first wavelength band limit.

FIG. 3 shows the gain characteristics of a Tm-doped optical fiberamplifier (TDFA).

FIG. 4 shown the gain characteristics of an Er-doped optical fiberamplifier (GS-EDFA).

FIG. 5 shows a block diagram of the structure of the third embodiment inthe first wavelength band limit.

FIG. 6 is a block diagram of the structure of the forth embodiment inthe first wavelength band limit.

FIG. 7 is a block diagram of the structure of the fifth embodiment inthe first wavelength band limit.

FIG. 8 shows the loss characteristics of an optical fiber installed at acite for communication use.

FIG. 9 shows the simulation results of the relation between wavelengthdispersion of an optical signal and the intensity of four-wave mixing.

FIG. 10 shows a typical example of the loss in the dispersion-shiftedfiber versus wavelength characteristics.

FIG. 11 shows the second wavelength band limit.

FIG. 12 shows the experimental results for the relation between averagetransmission power per channel an the power penalty.

FIG. 13 shows the second implementation in the second wavelength bandlimit.

FIG. 14 shows the third implementation in the second wavelength bandlimit.

FIG. 15 shows another example in the third implementation in the secondwavelength limit.

FIG. 16 shows a block diagram of the first wavelength divisionmultiplexed optical transmission system in the second wavelength bandlimit.

FIG. 17 shows a block diagram of the second wavelength divisionmultiplexed optical transmission system in the second wavelength bandlimit.

FIG. 18 shows a block diagram of the third wavelength divisionmultiplexed optical transmission system in the second wavelength bandlimit.

FIG. 19 shows a block diagram of the forth wavelength divisionmultiplexed optical transmission system in the second wavelength bandlimit.

FIG. 20 shows a block diagram of the fifth wavelength divisionmultiplexed optical transmission system in the second wavelength bandlimit.

FIG. 21 explains the wavelength band limit of the optical signal whenthe zero dispersion wavelength of the dispersion-shifted fiber is 1550nm.

PREFERRED EMBODIMENTS OF THE INVENTION

First, a wavelength division multiplexed optical transmission systemwherein the wavelength of wavelength division multiplexed opticalsignals is allocated between 1450 nm and 1510 nm or between 1570 nm and1610 nm, (this is called “the first wavelength band limit”) will beexplained for an optical transmission path a dispersion-shifted fiberdesigned so the zero dispersion wavelength is in the 1.55 μm region.

Following this, a wavelength division multiplexed optical transmissionsystem wherein the wavelength of wavelength division multiplexed opticalsignals are allocated between 1450 nm and 1530 nm or between 1570 nm and1650 nm (this is called “the second wavelength band limit”) will beexplained.

[Embodiment of the First Wavelength Band Limit]

Below, the reasons and system concept for setting the wavelengths ofwavelength division multiplexed optical signals between 1450 and 1510 orbetween 1570 and 1610 will first be explained. Following this, fiveembodiments of the system will each be explained.

The wavelength division multiplexed optical transmission system of thepresent invention uses as an optical transmission path adispersion-shifted fiber whose zero dispersion wavelength is set at 1.55μm, and each wavelength of optical signals is set so that the absolutevalue of the wavelength dispersion when propagating along withdispersion-shifted fiber is 0.5 ps/nm/km or greater.

In a citation (Fukui, et al. “Influence of fiber nonlinear effects onWDM transmission system with dispersion management,” 1996 GeneralConference of the Institute of Electronics, Information, andCommunication Engineers, 13-1138) the possible transmission distancelimit due to four-wave mixing was considerably extended if the absolutevalue of the dispersion is 0.5 ps/ns/km or above.

In contrast, the actual zero dispersion wavelength of adispersion-shifted fiber wherein the zero dispersion wavelength is setin the 1.55 μm region can be considered to be distributed from about1.535 μm˜1.565 μm when production variations are taken intoconsideration, but the wavelength dispersion value in the 1.55 μmwavelength region is almost a linear function of the wavelength. In thiscase, if the dispersion slope is +0.07 ps/nm²/km, a wavelength for whichthe absolute value of the wavelength dispersion is 0.5 ps/nm/km orgreater will be 1.53 μm or less, or 1.57 μm or greater.

In addition, in the present embodiment, as a wavelength band of thewavelength division multiplexed optical signals, one of these twowavelength band or both wavelength bands are used. Specifically, a 1.57μm to 1.61 μm wavelength bandwidth is used. Alternatively, a 1.45 μm to1.51 μm wavelength bandwidth is used. Additionally, both wavelengthbandwidths are used. Thus, due to a significant wavelength dispersionfor each wavelength, the necessary phase matching conditions forgenerating four-wave mixing are not satisfied, and the generation offour-wave mixing can be suppressed. As a result, it is possible toincrease the permissible optical input power of the dispersion-shiftedfiber, and the possible transmission distance can be greatly extended.

Conventionally, in optical fiber communication using the low loss regionof an optical fiber, the 1.55 μm band is used. The general reasons forthis are that during the initial period of optical fiber development, itwas reported that the low loss region was 1.55 μm, and in addition thatthe optical fiber amplifiers which brought about the remarkableadvancement in the capabilities of the optical communication systems inrecent years have an amplifier bandwidth at 1.55 μm. Therefore, inoptical fiber communication, the use of bands outside of the 1.55 μmband has not been neglected.

However, installed optical fibers on-site used for commination have theloss characteristics shown in FIG. 8. That is, in the wavelengthbandwidth of 1.57 μm -1.61 μm used in the present invention, it isapparent that there is an even lower loss above 1.55 μm. It is apparentfrom this that in addition to this effect, the effect can be obtainedthat it is possible to transmit at a lower loss by the use of wavelengthbandwidth conventionally neglected.

Moreover, in a wavelength division multiplexed optical transmissionsystem on which linear optical repeaters are disposed in the opticaltransmission path, either the optical signal in both wavelengthbandwidth can be amplified together by one optical amplifier, or theoptical signals of the various wavelength bandwidths can be separatedand amplified by separate optical amplifiers.

Next, five embodiments of the wavelength division multiplexed opticaltransmission system wherein the wavelengths of multiplexed opticalsignals are allocated either between 1450 nm and 1510 nm, or between1570 nm and 1610 nm.

(First Embodiment)

FIG. 1 shows the structure of the first embodiment of the presentinvention. The present embodiment shows an example of a nonrepeatedpoint-to-point transmission system which connects opposite an opticaltransmitter and optical receiver without repeaters.

In the figure, the present system comprises an optical transmitter 10,an optical transmission path 20, and an optical receiver 30. Moreover, asystem which directly modulates the bias, etc., of the light source canalso be applied to the optical transmitter 10.

The optical transmitter 10 here uses an external modulating system, andcomprises light sources 11 which set mutually differing wavelengths,modulators 12 which modulate the optical propagated wave output from thelight source by a data signal, an optical multiplexer 13 whichmultiplexes optical signals output from each modulator 12, and anoptical post-amplifier 14 which amplifies together the wavelengthdivision multiplexed optical signal output from the optical multiplexer13. Moreover, the optical post-amplifier 14 is provided as necessary.

The optical transmission path comprises a dispersion-shifted fiber 21whose zero dispersion wavelength is set at 1.55 μm.

The optical receiver 30 comprises an optical pre-amplifier 31 whichamplifies together the wavelength division multiplexed optical signalspropagated through the dispersion-shifted fiber 21, an opticaldemultiplexer 32 which demultiplexes the multiplexed optical signal torecover the optical signal of each wavelength, opto-electric converters(O/E) 33 which convert the optical signals of each wavelength toelectrical signals, and electrical reception circuits 34 which recoversthe data signal from each electrical signal. Moreover, if the opticalpre-amplifier is disposed before the opto-electrical converter 33, it ispossible to increase the receiver sensitivity. This opticalpre-amplifier and the optical pre-amplifier 32 in front of the opticaldemultiplexer 31 are installed as necessary.

The wavelength of the light source 11 is set so that the absolute valueof the wavelength dispersion when propagating along thedispersion-shifted fiber 21 is 0.5 ps/nm/km or greater. However, thezero dispersion wavelength of the dispersion-shifted fiber 21 is thoughtto be distributed approximately between 1.535 μm and 1.565 μm due tovariance during manufacture, but if the dispersion slope is +0.07ps/nm²/km, a wavelength whose absolute value of the wavelengthdispersion is 0.5 ps/nm/km or above becomes 1.53 μm or less, or 1.57 μmor greater. Therefore, the used wavelength band is a wavelength band of1.53 μm or below (for example, 1.45 μm˜1.51 μm), or the wavelength bandof 1.57 μm or greater (for example, 1.571 μm˜1.61 μm), or both of thewavelength bands.

(Second Embodiment)

FIG. 2 shows the structure of a second embodiment of the presentinvention.

The characteristic of the present embodiment is found in disposinglinear optical repeaters 22 along the optical transmission path which isa major structural component of an optical amplifier in order tocompensate the propagation loss of the dispersion-shifted fiber 21 inthe first embodiment. That is, this is an example of a multi-repeaterpoint to point propagation system. In this manner, the propagationdistance can be dramatically extended. The characteristics of thedispersion-shifted fiber 21 and the used wavelength band are the same asthose of the first embodiment. The propagation distance like that in thepresent embodiment is long, and when the optical power is maintained ata high level by linear optical repeaters 22, in the conventionalconstruction the degradation of the transmission quality due tofour-wave mixing is severe, but in the structure which limits the usedwavelength band as in the present invention, the influence is small, andthe effect remarkable.

In the first and second embodiments, as an optical amplifier, an opticalfiber amplifier or a semi-conductor laser amplifier can be used, but anappropriate structure should be chosen depending on the variouswavebands used.

As an optical amplifier for the 1.45 μm˜1.51 μm band, a Tm-doped opticalfiber amplifier (TDFA) can be used. With respect to gaincharacteristics, as shown in FIG. 3, the high gain region is in the 1.45μm˜1.48 μm band. In exciting this TDFA, a light source in the 1.0 μm˜1.2μm band is used. As the excitation light source in this wavelength band,presently, an Nd: YAG laser and an Nd: YLF laser are available. Inaddition, by structuring the optical fibers for amplification in acascade connection via an isolator or an optical band pass filter, aneven higher gain amplifier can be obtained.

As an optical amplifier for the 1.57 μm˜1.61 μm band, an Er-doped gainshifted optical fiber amplifier (GS-EDFA) is available. By optimizingthe Er density, etc., of the optical fiber for amplification, it canshift the gain region (1.53 μm˜1.56 μm) of the typical EDFA. Its gaincharacteristics are shown in FIG. 4. In the excitation of this GS-EDFA,a light source in the 0.98 μm or 1.48 μm neighborhood is used.

When the 1.53 μm or less wavelength band and the 1.57 μm or greaterwavelength band are used simultaneously, by using a semiconductor laseramplifier with a wide gain bandwidth, it is possible to amplify theoptical signals of both wavelength bands together. In addition, thedevelopment of optical fiber amplifiers which can amplify the opticalsignals in both wavelength bands together is progressing. Additionally,the respective optical signals of both wavelength bands can bemultiplexed after being amplified separately. An example of thisstructure is explained in the third embodiment which follows.

(Third Embodiment)

FIG. 5 shows the structure of the third embodiment. This embodiment isan example of the multi-repeater point-to-point transmission systemwhich is similar to that of the second embodiment shown in FIG. 2. Thosefunctions which are the same as those in FIG. 2 have the same referencenumbers.

In the optical transmitter 10, the optical signal in the 1.45 μm˜1.51 μmband is amplified by an optical post-amplifier 14A using a TDFA, such asthe one shown in FIG. 3, and the optical signal in the 1.57 μm˜1.61 μmband is amplified by an optical post-amplifier 14B using a GS-EDFA, suchas the one shown in FIG. 4. In addition, the optical signals of bothbandwidths are multiplexed by a bandwidth multiplexer WDM filter 41, andtransmitted to the dispersion-shifted fiber 21.

In the linear optical repeater 22, the optical signals of both bands aredemultiplexed by a bandwidth demultiplexer WDM filter 42, the opticalsignal in the 1.45 μm˜1.51 μm band is amplified by an optical amplifier43A using a TDFA, such as that shown in FIG. 3, the optical signal inthe 1.57 μm˜1.61 μm band is amplified by an optical amplifier 43B usinga GS-EDFA, such as that shown in FIG. 4, and then the optical signals inboth wavelength bands are multiplexed again by a band multiplexer WDMfilter 41.

In optical receiver 30, the optical signals of both bands aredemultiplexed by a bandwidth demultiplexer WDM filter 42, the opticalsignals in the 1.45 μm˜1.51 μm band are amplified by an opticalamplifier 31A using a TDFA, such as that shown in FIG. 3, and theoptical signal in the 1.57 μm˜1.61 μm band is amplified by an opticalamplifier 31B using a GS-EDFA, such as that shown in FIG. 4. Below, eachoptical signal is demodulated in the same manner as in the secondembodiment.

The first, second, and third embodiments described above are examples ofa point-to-point transmission system, and explained that degradation ofthe propagation quality due to four-wave mixing is avoid, and thetransmission distance of the system using dispersion-shifted fibers canbe dramatically increased. However, the present invention is not limitedto a point-to-point transmission system, and can be applied to allwavelength division multiplexed optical transmission systems of anetwork. For example, it can be applied to a multi-repeater opticaltransmission on which signals which have been transformed intoelectrical signals after being demultiplexed in the optical receiver 30in the second embodiment are digitally demodulated, and on which, ifnecessary, electrical signals are transformed into optical signals,wavelength division multiplexed, and transmitted to the opticaltransmission path after conducting routing processing electrically, oron which this procedure is repeated many times.

Finally, a wavelength division multiplexed optical transmission systemon which an optical node that adds or drops one or specified opticalsignals along the optical transmission path can also be applied.Examples of this structure are explained below as the forth and fifthembodiments.

(Fourth Embodiment)

FIG. 6 shows the structure of the fourth embodiment of the presentinvention.

In the figure, this system comprises a center node 50, remote nodes 60,and a dispersion-shifted fiber 21 which connects them in a ring.

Each remote node 60 carries out communication with the center node 50 byat least one or more differing wavelengths being allocated, and usingeach wavelength. A remote node 60 comprises an optical pre-amplifier 61disposed as necessary, an optical adding and dropping circuit 62 whichadds from the wavelength division multiplexed optical signals theoptical signal with the allocated wavelength, and feeds the opticalsignal of this wavelength into the wavelength division multiplexingoptical signals, and an optical post-amplifier 63 installed asnecessary.

The center node 50 comprises a transmission system further comprising anoptical transmitter 51 for each corresponding wavelength allocated toeach remote node, an optical multiplexer 52 which multiplexes theoptical signal of each wavelength, an optical post-amplifier 53installed as necessary, and a receiving system comprising an opticalpre-amplifier 54 installed as necessary, an optical demultiplexer 55which demultiplexes the wavelength division multiplexed signals into theoptical signals of each wavelength, and an optical receiver 56 for eachcorresponding wavelength.

The wavelength division multiplexed optical signals multiplexed by thecenter node 50 arrive at the remote nodes 60 by being propagated alongthe dispersion-shifted fiber 21. In the remote nodes 60, only theoptical signals with the allocated wavelength are dropped off from thewavelength division multiplexed optical signals, and then the opticalsignals of this wavelength are added into the wavelength divisionmultiplexed optical signals. The optical signals which have transitedeach remote node 60 arrive at the center node 50, and are dropped offhere with each wavelength. In this manner, the structure of thisembodiment is physically a ring network structure, but logically, it isa star network structure in which the center node 50 and remote nodes 60are connected in a star formation by a bus which is distinguished bywavelength. Along the dispersion-shifted fiber 21, linear opticalrepeaters which compensate the transmission loss can be feed asnecessary.

(Fifth Embodiment)

FIG. 7 shows the structure of the fifth embodiment of the presentinvention.

The characteristics of the present embodiment are that the center node,which gathers information, is eliminated from the structure of thefourth embodiment, paths which allocate dedicated wavelengths betweeneach remote node are formed, and each remote node is connected by a meshformation.

A remote node 60 comprises an optical pre-amplifier 61 which isinstalled as necessary, an optical adding and dropping circuit 62 whichdrops off the optical signals of an allocated wavelength from thewavelength division multiplexed optical signals, and adds the opticalsignals of this wavelength into the wavelength division multiplexedoptical signals, and an optical post-amplifier 63 which is installed asnecessary. In the communication between each remote node, variouswavelengths are allocated, and if, for example, the total number ofremote nodes is given as N, the remote node #1 carries out communicationwith remote nodes #2, #3, . . . , #N by using the signals of wavelengthsλ12, 13λ, . . . : λ1N. In the case of propagation with one opticalfiber, N (N−1)/2 wavelengths are necessary. If two optical fibers areused, it is possible to decrease the number of wavelengths by about(N−1)/8. Along the dispersion-shifted fiber 21, linear optical repeaterswhich compensate transmission loss can be feed as necessary.

If the present inversion is applied to the wavelength division multiplexlink net which is formed with dispersion-shifted fibers as shown in thefourth embodiment or the fifth embodiment, it is possible to avoid theinfluence of four-wave mixing, expand the transmission distance betweennodes, minimize channel separation, easily increase of the number ofchannels, and a significant effect can be expected. For example, the1.45 μm˜1.51 μm band, the 1.57 μm˜1.61 μm band, or both bands can beused as the used wavelength bandwidth. The linear optical repeaters,when using both wavelength bands, can be similar to these in the thirdembodiment.

As explained above, the wavelength division multiplexed opticaltransmission system of the present invention can avoid the influence offour-wave mixing in a dispersion-shifted fiber by limiting the usedwavelength bandwidth. In this manner, it is possible to increase thepermissible input power of the dispersion-shifted fiber, and it ispossible to extend greatly the potential transmission distance.

In addition, when using the 1.57 μm˜1.61 μm bandwidth, becausetransmission loss can be even further decreased from the 1.55 μm band,it is possible to extend the possible transmission distance beyond thatof the conventional 1.55 μm band. [Embodiment Related to the Restrictionof the Second Wavelength Band]

Below, a wavelength division multiplexed transmission system wherein thewavelength of multiplexed optical signals are allocated either from 1450nm to 1530 nm, or 1570 nm to 1650 nm will be explained.

First, the reason that the wavelength of the multiplexed optical signalis between 1450 nm and 1530 nm or between 1570 nm and 1650 nm will beexplained.

To begin with, the relation between the wavelength dispersion of theoptical signal, and the optical intensity of the four-wave mixing willbe explained.

Here, as explained above, “four-wave mixing” is a phenomenon wherein anew frequency f_(FWM)=f_(i)+f_(j)−f_(k) of the four-wave mixing aregenerated from non-linear interaction between three frequencies f₁, f₂,and f₃, and the propagation medium. Here, i, j, and k take any valuefrom 1 to 3, and j≠k. A four-wave mixing can occur when f_(i) equalsf_(j), that is, even when two frequencies are launched. In wavelengthdivision multiplex communication using a wavelength region with smalldispersion, the generation efficiency of the four-wave mixing increasesas the amount of phase matching Δβ becomes smaller. Here, the amount ofphase mismatching is expressed by:

Δβ=(−λ⁴ π/c ²)·(dD/dλ)·{(f _(i) −f ₀)+(f _(j) −f ₀)}·(f _(i) −f _(k))·(f_(j) −f _(k)),

and is described in K. Inoue, “Fiber four-wave mixing in thezero-dispersion wavelength region,” J. Lightwave Technology, Vol. 10,pp. 1553-1561, 1992.) Here, f₀ is the zero dispersion frequency of thefiber. In addition, λ is the wavelength, c is the speed of light, and Dexpresses the wavelength dispersion. From this equation, it can beunderstood that Δβ becomes zero when among the launched wavelengthmultiplexed signals, the optical frequency of one optical signal agreeswith f₀(f_(i)=f_(j)=f₀), or when f₀ lies between the optical frequenciesof two optical signals (f_(i)−f₀=f₀−f_(j)), and the generationefficiency of four-wave mixing is at its highest. When the differencebetween the frequency of the generated four-wave mixing and any of theoptical frequencies of the optical signals is within the receivingbandwidth of the receiver, the four-wave mixing waves becomeinterference noise to the optical signals. When the frequencies of theoptical signal disposed on an equally spaced optical frequency grid areallotted, that is, in the case of an equally spaced optical frequencyallocation, the optical frequency of the generated four-wave mixingalways come to be positioned on these grids. Because of this, in thecase of an equally spaced optical frequency allocation, the influence ofinterference noise due to four-wave mixing is severe.

FIG. 9 is the result of simulation of the relation between thewavelength dispersion of a signal and the optical intensity of thefour-wave mixing. The conditions of the simulation are as recorded inthe upper right of FIG. 9. Moreover, the power of the four-wave mixingin this simulation was calculated according to the method presented inK. Inoue, H. Toba, “Fiber four-wave mixing in multi-repeater systemswith nonuniform chromatic dispersion.” J. Lightwave Technology, 13, pp.88-93, 1995.

In FIG. 9, among 16 optical signals spaced at 200 GHz, the wavelengthdispersion of the optical signal of the channel with the leastwavelength dispersion is the abscissa, and when these signals arepropagated along an optical fiber, the ratio (dB) of the four-wavemixing intensity to the signal intensity is shown on the ordinate. It isknown that when the ratio of the optical intensity of the four-wavemixing to the optical signal intensity is −30 dB or greater, the opticalsignal deteriorates, and from FIG. 9, it can be seen that when theoptical dispersion of a signal nearest the zero dispersion wavelength is0.35 ps/km/nm or less, degradation becomes great. As described above,generally because the dispersion slope of an optical fiber is about 0.07ps/nm²/km, when the wavelength distance from the zero dispersionwavelength of the optical signal whose optical dispersion is nearest thezero dispersion wavelength falls below 5 nm (=0.35/0.07), there isdegradation. In other words, if the wavelength distance from the zerodispersion wavelength of the optical signal whose optical dispersion isnearest the zero dispersion wavelength is 5 nm or greater, the problemsdue to four-wave mixing can be avoided.

In addition, due to variance in manufacturing, presently the zerodispersion wavelength of the dispersion-shifted fibers widelymanufactured and installed is generally distributed from 1535 nm to 1565nm, centered on 1550 nm. Thus, when dispersion-shifted fibers presentlymanufactured and installed are used as the optical transmission path,the problem of degradation due to four-wave mixing can be avoided bymaking the wavelength of the signal 1530 nm (1535 nm−5 nm) or less, or1570 nm (1565 nm+5 nm) or greater.

Next, the reason for setting the range of the wavelength of the opticalsignal from 1450 nm to 1650 will be explained.

FIG. 10 shows a typical example of the characteristics of thedispersion-shifted fiber loss versus wavelength. When using adispersion-shifted fiber as the optical transmission path, the span isgenerally 100 km. In addition, the gain of the optical amplifiers whichform the repeaters is generally 30 dB. Here, it can be understood thatif the fiber loss is 0.3 dB/km (30/100), a wavelength form 1450 nm to1650 nm can be used.

From the above, when dispersion-shifted fibers presently manufacturedand installed are used as an optical transmission path, if thewavelengths between 1450 nm and 1530 nm shown by reference numeral 112in FIG. 11, or the wavelength between 1570 nm and 1650 nm shown byreference numeral 111 are used as the wavelength of the optical signal,there is no degradation in transmission loss incurred due to four-wavemixing, and long distance wave multiplexed transmission can be realized.Moreover, reference numeral 110 denotes the zero dispersion wavelengthdistribution of the dispersion-shifted fiber which is the opticaltransmission path.

Next, the validity of the wavelength bands 111 and 112 of the opticalsignal shown in FIG. 11 is demonstrated experimentally. FIG. 12 is theexperimental results demonstrating the relation between the averagetransmission intensity for each channel and the power penalty. In thefigure, the abscissa is the average transmission output intensity foreach channel, and the ordinate is the power penalty due to four-wavemixing. The dispersion-shifted fiber used had a length of 40 km, atransmission optical signal bit rate of 10 Gb/s, 8 differentwavelengths, and an optical frequency spacing of 200 GHz. Here, thewavelengths of the optical signals in the experiment were the twoconventionally used 1543˜1556 nm wavelength band, and the 1581˜1589 nmwavelength band according to the present embodiment.

The power penalty used here is defined as follows:

power penalty [dB]=10×log (Pt/Pb).

Here, Pb is the average received optical power necessary to achieve abit error rate of 10⁻⁹ when the transmitter is connected directly to thereceiver and there is no transmission over a dispersion-shifted fiberused as an optical transmission path. In addition, Pt is the averagereceived optical power necessary to achieve a bit error rate of 10⁻⁹after a 40 km transmission along a dispersion-shifted fiber.

As is clear from this figure, when using the conventionally used1543˜1556 nm as the wavelength of the optical signals, when thetransmission intensity per channel is increased, the power penalty dueto the influence of four-wave mixing increases.

In contrast, when the 1581˜1593 nm wavelength band according to thepresent embodiment is used, because the influence of four-wave mixingdoes not become a problem, the power penalty does not increase. Inaddition, the possibility of increasing the transmission intensity perchannel implies the possibility of increasing the input power of thelinear repeaters, and because of this, it is possible to decrease theinfluence of noise of the optical amplifier, and the transmissiondistance, that is, the span between repeaters, can be lengthened.

Next, implementations of the wavelength band when the wavelength of thesignal is either between 1450 nm and 1530 nm or between 1570 nm and 1650nm is explained.

(First Implementation)

There is an implementation of a wavelength band wherein the wavelengthof the optical signal is allotted to the long wavelength area which canavoid the problems caused by four-wave mixing, that is, allotted to the1570 nm 1650 nm, shown by the reference number 111 in FIG. 11.

In addition, there is an implementation of a wavelength band wherein thewavelength of the optical signal is allotted to the short wavelengtharea which can avoid the problems caused by four-wave mixing, that is,allotted to the 1450 nm˜1530 nm, shown by the reference number 112 inFIG. 11.

Furthermore, it is also possible to use wavelengths between 1450 nm and1530 nm, and between 1570 nm and 1650 nm as the wavelength of theoptical signal. It is possible to increase the transmission capacity ofthe fiber in this case.

Moreover, in the above implementation, there is no limitation on thepropagation direction of the optical signal. Therefore, all the opticalsignals can propagate in the same direction, or one part of the opticalsignals can propagate in a different direction from another opticalsignals.

(Second Implementation)

In the second implementation, the propagation direction of the opticaltransmission path of the optical signals whose wavelengths are between1450 nm and 1530 nm shown by reference numeral 112 in FIG. 13 and thepropagation direction of the optical transmission path of the opticalsignals whose wavelengths are between 1570 nm and 1650 nm shown byreference numeral 111 are opposite directions. Below, the reason formaking the form in this manner will be explained.

In the first implementation it was assumed that the wavelengths of theoptical signals between 1450 nm and 1530 nm and between 1570 nm and 1650nm are used simultaneously, and that the propagation directions of alloptical signals are the same. In this case, when the bit rate of eachwavelength is comparatively small, the walk-off between the opticalsignals between 1450 nm and 1530 nm and the optical signals between 1570nm and 1650 nm is of the same order as the time for one time slot. As aresult, cross-talk due to stimulated Raman scattering is produced, andthe problem of degradation of the transmission quality cannot beignored. Moreover, “walk-off” means that as a result of a difference ingroup delay time, the relative time position of two optical signals ofdiffering wavelength become separated depending of the fiberpropagation. In addition, “stimulated Raman scattering” is thephenomenon wherein the energy of a short-wave signal is transferred to along wavelength signal through the vibration of molecules which form thefiber.

Because stimulated Raman scattering is produced only when opticalsignals in the short-wave part and optical signals in the long-wave partexist together, the decrease in the power of the short-wave signalchanges depending on the combination of the signs of both signals andthe relative time position, and this causes cross-talk, producing adegradation in the transmission characteristics. In order to avoid thisproblem, propagating the wavelengths between 1450 nm and 1530 nm and thewavelengths between 1570 nm and 1650 nm in opposite directions iseffective. This is because the walk-off of the short-wave signal and thelong-wave signal increases, and it is possible to average out thedecrease in power of the signal in the short-wave part due to stimulatedRaman scattering.

This kind of bi-directional transmission is also useful for avoidingcross-talk due to degenerate four-wave mixing produced between theoptical signals between 1450 nm and 1530 nm and the optical signalsbetween 1570 nm and 1650 nm, and waveform degradation due to cross phasemodulation. This is because the walk-off due to bi-directionaltransmission increases, the phase matching conditions for degeneratefour-wave mixing are not satisfied, and the cross phase modulation isaveraged. Here, cross-phase-modulation is a phenomenon where the phaseof an optical signal changes due to a change in the local refractionindex caused by other optical signal.

For these reasons, it is preferred that the propagation direction of theoptical transmission path of the optical signal between 1450 nm and 1530nm and the propagation direction of the optical transmission path of theoptical signal between 1570 nm and 1650 nm be opposite.

(Third Implementation)

Above, the case wherein the wavelengths of the optical signals areallocated between 1450 nm and 1530 nm, and between 1570 nm and 1650 nmwas described. Here, the wavelength band between 1530 nm and 1570 nm isnot used in order to avoid signal degradation due to the above-describedfour-wave mixing. Signal degradation due to four-wave mixing, asdescribed in detail in the above-mentioned reports, can be suppressed byunequal channel spacing of wavelengths arrangement.

Here, in “unequal spacing wavelength arrangement” means the differencebetween the frequency f_(FWM)=f_(i)+f_(j)+f_(k) of the four-wave mixing,which is generated from the arbitrary three waves of optical frequenciesf₁, f₂, and f₃, and either of is f₁, f₂, and f₃ is greater than thereceiving bandwidth of the receiver, so they are arranged in such a waythat the optical frequency differences between each optical signal haveunequally spaced channels. Here, i, j, and k have a value from 1 to 3,and j≠k. For example, the four-wave mixing waves, which three arbitrarywaves among 12 wavelength division multiplexed optical signals whosefrequency intervals are allotted at 135, 300, 375, 150, 175, 350, 250,150, 325, and 225 GHz, is generates at wavelength frequency separated byat least by 25 GHz from any signals, and does not become interferencenoise.

In this context, in an optical signal having a wavelength near the zerodispersion wavelength of the dispersion-shifted fiber which is theoptical transmission path, it is possible to expand one part of theuseable wavelength range by using an unequal channel spacing frequencyallocation.

One example of this implementation is shown in FIG. 14. The wavelengthof wavelength multiplexed optical signals are allocated between 1450 nmand 1570 nm, shown by reference numeral 120, and between 1570 nm and1650 nm, shown by reference numeral 111. In addition, the opticalsignals whose wavelengths are allocated between 1450 nm and 1570 nm andthe optical signals distributed between 1570 nm and 1650 nm arepropagated along the dispersion-shifted fiber propagation path inopposite directions. Even in the worst case in which the fiber's zerodispersion wavelength is 1535 nm, in order to avoid degradation due tofour-wave mixing, at least the optical frequency difference between theoptical signals above 1505 nm (=1535−(1565−1535)) or greater and 1565 nmor below shown by reference numeral 130 are allocated with unequalspacing.

Moreover, among the optical signals distributed between 1450 nm and 1570nm shown by reference numeral 120, when the wavelengths of the opticalsignals among them are near 1570 nm, the optical frequency differencesof 1500 nm (=1535−(1570−1535)) or greater and 1570 or less are allocatedwith unequal spacing.

In this manner, when divided between the 1450 nm and 1530 nm shown byreference numeral 112 and between 1530 nm and 1650 nm shown by referencenumeral 121 in FIG. 15, the optical signals whose wavelengths aredistributed between 1450 and 1530 nm, and the optical signals whosewavelengths are distributed between 1530 nm and 1650 nm are propagatedin the opposite direction along the dispersion-shifted fibertransmission path. In addition, even in the worst case in which the zerodispersion wavelength of the fiber is 1565 nm, in order to avoiddegradation due to four-wave mixing, at least the optical frequencydifference between the optical signals with wavelengths, shown byreference numeral 131, between 1535 nm or greater and 1595 nm(=1565+(1545−1535)) or less are allocated with unequal spacing.

Moreover, among the optical signals allocated between 1530 nm and 1650nm, shown by reference numeral 121, when the wavelength of the opticalsignals among them are close to 1530 m, the optical frequency differencebetween optical signalss 1530 nm or greater and 1600 nm(=1545+(545−1530)) or less are distributed with unequal spacing.

Next, examples of a wavelength division multiplexed optical transmissionsystem wherein the wavelength band used in the optical signals explainedin the first through third embodiments is limited will be explained withreference to FIGS. 16 to 20.

FIG. 16 is a block diagram of the first wavelength division multiplexedoptical transmission system. According to FIG. 16, this system comprisesoptical transmission and reception apparatuses 212 and 213 in turncomprising a transmission circuit 210 and a reception circuit 121, andone optical fiber transmission path 224 connecting the opticaltransmission and reception apparatuses 212 and 213. The transmissioncircuit 210 comprises transmitters 220 which generate optical signals ofdifferent wavelengths and a multiplexer 221 which wavelength multiplexesoptical signals, and the reception circuit 211 comprises a demultiplexer222 which separates the optical signals, and receivers 223 whichdemodulate the electrical signal from the demultiplexed optical signals.In addition, the optical transmission and reception apparatuses 212 and213 comprise the transmission circuit 211 and the reception circuit 212,and a filter or circulator 225. Moreover, the transmitter 220 comprisesan optical source 11 which sets the different wavelengths in FIG. 1, anda modulator 12 which modulates the optical transmission wave output fromthe light source with a data signal, and the receiver 223 comprises theopto-electric converter (O/E) in FIG. 1, and an electrical receptioncircuit 34 which demodulates the data signal from each electricalsignal.

FIG. 17 is a block diagram of the second wavelength division multiplexedoptical transmission system. Compared with the system in FIG. 16, thisis a point-to-point nonrepeated or multi- repeater optical wavelengthdivision multiplexed bi-directional transmission system characterized inamplifying together all optical signals by at least one bi-directionaloptical amplifier 270, when transmitted, received, or repeated.Moreover, the parts in FIGS. 17 to 20 which correspond to parts of FIG.16 have identical reference numerals, and their explanation is omitted.

In this manner, in a system which transmits optical signals on anoptical fiber transmission path 224 with high power using abi-directional optical amplifier 270, the degradation due to four-wavemixing and Raman cross-talk becomes great in the conventional wavelengthbands on which optical signals are distributed. However, by using theimplementation of wavelengths of optical signals described above, thesecan be avoided.

FIG. 18 is a block diagram showing the third wavelength divisionmultiplexed optical transmission system. This system is characterized inthe optical signals being amplified by the optical amplifiers 280 and281 depending on the differences in their propagation directions whenseparated by the differences in their propagation direction by a filteror circulator 225 when transmitted, received, or repeated.

This system, like the system shown in FIG. 17, transmits an opticalsignal along an optical fiber transmission path at high power using anoptical amplifier, and the effect from the implementation of thewavelengths of the optical signal described above are great.

FIG. 19 is a block diagram showing the fourth wavelength divisionmultiplexed optical transmission system. This system, in comparison withthe systems of FIG. 17 and FIG. 18, is characterized in that before orafter being propagated along the optical fiber transmission path 224,all optical signals are dispersion compensated together by a dispersioncompensating fiber 290 possessing a dispersion slope with the reversesign of this optical fiber transmission path 224, and whose zerodispersion wavelengths are almost equal.

FIG. 20 shows a block diagram of the fifth wavelength divisionmultiplexed optical transmission system. This system, in comparison withthe systems of FIG. 17 and FIG. 18, optical signals are each dispersioncompensated by dispersion compensation fibers 2100 and 2102 which havean average dispersion equal to that of the optical signals in theirrespective propagation directions, and have a dispersion slope withreverse sign when optical signals are separated depending on theirpropagation direction by a filter or circulator 225 when transmitted,received, or relayed.

Moreover, in the first implementation of the above-described wavelengthband, when the propagation direction of all optical signals is the same,embodiments one through five explained with the first wavelength bandlimitation are also satisfactory.

In addition, the third implementation in the wavelength distribution ofthe optical signal described above is not limited to point-to-pointtransmission, but can be adapted to all network wavelength divisionmultiplexed optical transmission systems.

Moreover, in the present embodiment, because it is assumed that widelyfabricated and installed dispersion-shifted fibers are used as anoptical transmission path, the above-described implementation three canbe used for the wavelength distribution of the optical signal. Thistechnological conception can be applied to an optical transmission pathwhich has characteristics differing from those of the dispersion-shiftedfiber described above.

For example, if the zero dispersion wavelength of the dispersion-shiftedfiber which is an optical transmission path is, for example, 1550 nm, asshown in FIG. 21, the wavelengths over which the optical signals aredistributed can be either between 1450 nm and 1545 nm (1550−5) orbetween 1555 nm (1550+5) and 1650 nm. That is, depending on thecharacteristics of the optical transmission path, the wavelength bandover with the optical signals are distributed can be decided based onthe above-described technological conception.

As described above, by limiting the wavelength band used in multiplexedoptical signals, in an already installed dispersion-shifted fiber, it ispossible to avoid four-wave mixing. Thus, it is possible to increase thepermissible input power to the dispersion-shifted fiber, and greatlyextend the possible transmission distance.

What is claimed is:
 1. A wavelength division multiplexed opticaltransmission system comprising a dispersion-shifted fiber whosezero-dispersion wavelength is in the 1550 nm region, wherein: amongwavelength multiplexed optical signals, the wavelengths of either of atleast two optical signals are allocated between 1450 nm and 1530 nm, orbetween 1570 and 1650 nm.
 2. A wavelength division multiplexed opticaltransmission system according to claim 1 wherein: among said wavelengthmultiplexed optical signals, the wavelengths of at least two of theoptical signals are allocated between 1450 nm and 1530 nm.
 3. Awavelength division multiplexed optical transmission system according toclaim 1 wherein: among said plurality of wavelength multiplexed opticalsignals, the wavelengths of at least two of the optical signals areallocated between 1570 nm and 1650 nm.
 4. A wavelength divisionmultiplexed optical transmission system according to claim 1 wherein:among wavelength multiplexed optical signals, the wavelengths of eitherof at least two optical signals are allocated between 1450 nm and 1530nm, and between 1570 and 1650 nm.
 5. A wavelength division multiplexedoptical transmission system according to claim 4 wherein: the opticalsignal whose wavelength is allocated between 1450 nm and 1530 nm and theoptical signal whose wavelength is allocated between 1570 nm and 1650 nmpropagate along said dispersion-shifted fiber in opposite directions. 6.A wavelength division multiplexed optical transmission system comprisinga dispersion-shifted fiber whose zero-dispersion wavelength is in the1550 nm region, wherein: the wavelength of multiplexed optical signalsare allocated between 1450 nm and 1570 nm, and 1570 nm and 1650 nm, theoptical signal whose wavelength is allocated between 1450 nm and 1570nm, and the optical signal whose wavelength is allocated between 1570 nmand 1650 nm propagate along said dispersion-shifted fiber in oppositedirections, and at least the optical wavelength differences of theoptical signals whose wavelength is 1505 nm or greater and 1565 or lessare unequally spaced.
 7. A wavelength division multiplexed opticaltransmission system comprising a dispersion-shifted fiber whosezero-dispersion wavelength is in the 1550 nm region, wherein: thewavelengths of said plurality of multiplexed optical signals areallocated between 1450 nm and 1530 nm and between 1530 nm and 1650 nm,the optical signal whose wavelength is allocated between 1450 nm and1530 nm, and the optical signal whose wavelength is allocated between1530 nm and 1650 nm propagate along the dispersion-shifted fiber inopposite directions, and at least the optical wavelength differences ofthe optical signals whose wavelength is 1535 nm or greater and 1595 orless are unequally spaced.
 8. A wavelength division multiplexed opticaltransmission method in which a dispersion-shifted fiber whose zerodispersion wavelength is in the 1550 nm region is a transmission path,wherein: among wavelength multiplexed optical signals, the wavelengthsof either of at least two optical signals are either allocated between1450 nm and 1530 nm, or between 1570 and 1650 nm.
 9. A wavelengthdivision multiplexed optical transmission system comprising adispersion-shifted fiber whose zero-dispersion wavelength is in the 1550nm region, wherein at least two wavelength multiplexed optical signalshave their wavelength allocated in at least one of first and secondwavelength regions, the first and second wavelength regions beingbetween 1450 nm and 1530 nm and between 1570 nm and 1650 nm,respectively.
 10. A wavelength division multiplexed optical transmissionsystem comprising a dispersion-shifted fiber whose zero-dispersionwavelength is in the 1550 nm region, wherein: the wavelengths ofmultiplexed optical signals are allocated between 1450 nm and 1670 nmand between 1570 nm and 1350 nm; the optical signals whose wavelength isallocated between 1450 nm and 1570 nm and whose wavelength is allocatedbetween 1570 nm and 1650 nm propagate along said dispersion-shiftedfiber in opposite directions; and at least the optical wavelengthdifferences of the optical signals whose wavelength is 1505 nm orgreater and 1565 nm or less are unequally spaced.
 11. A wavelengthdivision multiplexed optical transmission system comprising adispersion-shifted fiber whose zero-dispersion wavelength is in the 1550nm region, wherein; the wavelengths of multiplexed optical signals areallocated between 1450 nm and 1530 nm and between 1530 nm and 1650 nm;the optical signals whose wavelength is allocated between 1450 nm and1530 nm and whose wavelength is allocated between 1530 nm and 1650 nmpropagate along the dispersion-shifted fiber in opposite directions; andat least the optical wavelength differences of the optical signals whosewavelength is 1535 nm or greater and 1595 or less are unequally spaced.